Typical cyclic processors are injection moulding machines, blow moulding machines and aluminium die casting machines. For convenience the invention will be described in relation to thermoplastic injection moulding.
In thermoplastic injection moulding machines the hot melt material is introduced, typically injected, into a mould cavity, which cavity is then cooled in order that the material will solidify to form the product to a shape dictated by the mould cavity; the mould is thereafter opened, the formed product ejected, the mould closed to re-make the cavity, and the cycle repeated. For efficient working, the mould operating temperature must be held within an acceptable range defined by specified upper and lower temperature limits, since if the mould temperature is too high the material is slow to solidify, and if the mould temperature is too low some or all of the mould material can solidify in the injection sprue leading to the cavity i.e. premature solidification with a consequent increase in the required injection pump pressure. Furthermore it is considered good practice to maintain the mould within a temperature range which allows the product the minimum shrinkage and distortion during the setting or curing stage, both for product quality and for uniformity amongst the replications of the product being moulded.
The four variables which affect product quality are melt temperature, melt flow rate, melt pressure (each a function of the processor control systems and condition settings) and cooling rate. It is normal practice to seek to maintain product quality by repeated small adjustments of the processor machine controls, usually in response to external changes "outside" the machine controls. A reliable and accurate temperature regulating system is necessary so that the operating temperature, and therefore the cooling rate, can be held within a pre-determined range.